Spirometry, i.e. the measurement of inspired and expired air flows and volumes at the airway opening of a test subject, is the commonest means of assessing lung function. Diseases causing obstruction of larger airways will eventually result in reduced expiratory flows and volumes as measured by spirometry. However, spirometry findings are often normal in the early stages of peripheral airway diseases, and therefore changes (obstruction or restriction) in the peripheral airways associated with early cystic fibrosis disease, early chronic obstructive pulmonary disease or mild asthma cannot be detected by spirometry until the disease has progressed considerably because the small airways only contribute to a very limited extent to the total airway resistance.
Peripheral airway diseases do, however, affect the way air mixes within the lungs and thus lead to increased ventilation inhomogeneity. Ventilation inhomogeneity may be assessed using the multiple-breath wash-out (MBW) test performed by washing out either a resident gas (e.g. nitrogen) from the lungs using e.g. pure oxygen or a previously washed-in non-resident inert tracer gas during tidal breathing of room air.
When using the MBW method to assess ventilation distribution in the lungs, several indices of overall ventilation inhomogeneity can be calculated as sensitive tracers of airway disease. They all reflect differences in specific ventilation between large and/or relatively small lung regions, resulting in delayed wash-out of the tracer gas from the poorly ventilated regions. Examples of indices of overall ventilation inhomogeneity that can be calculated from the MBW are mixing ratio (MR), which is calculated as the ratio between the actual and the estimated ideal number of breaths needed to lower the end-tidal tracer gas concentration to a certain fraction of the starting value, moment ratios of the wash-out curve and lung clearance index (LCI). The LCI can be calculated as the cumulative expired volume (VCE) required to clear the gas from the lungs minus the number of wash-out breaths multiplied by external dead space outside the lips, divided by the subject's FRC (up to the lips). FRC is the amount of air that stays in the lungs after a normal expiration. In other words, LCI represents the number of lung volume turnovers (i.e. FRCs) that the subject must breathe to clear the lungs from the tracer gas (by convention, to an end-tidal concentration of 1/40th of the starting concentration over three subsequent breaths). Disregarding the correction for external dead space the equation is:
                    LCI        =                              V            CE                    FRC                                    (        1        )            
The LCI is simple to calculate and intuitively understandable, and it is questionable whether any other index is more sensitive, reliable and clinically useful.
For the MBW test using a non-resident inert tracer gas there are several different gases with low solubility in blood and tissues that can be used including helium (He) and sulfur hexafluoride (SF6). Normally, a respiratory mass spectrometer is used for the gas analysis, and a flowmeter, e.g. a pneumotachometer, is used to record the inspiratory and expiratory flows at the mouth. The pressure gradient measured is directly related to flow thus allowing a computer to derive a flow-curve measured in L/minute. The test subject is breathing through a mouthpiece or a face mask connected to the flowmeter, and a gas sampling tube is connected to the breathing assembly for sidestream gas analysis. When performing MBW tests by use of a non-resident tracer gas, the tracer gas must first be washed in to obtain an even concentration in the lungs before the wash-out can start. A conventional breath-by-breath system for inert gas wash-out therefore further consists of a unit for delivering the wash-in gas mixture. This can be achieved by use of a bias flow of the gas mixture. A sufficiently long wash-in period is needed to allow the tracer gas to fully equilibrate in the lungs, which may take quite some time. Conventional wash-in by use of a bias flow also results in a significant consumption of gas because the bias flow must exceed the peak inspiratory flow of the test subject. Alternative setups, such as a demand valve, can be used but may lead to increased external dead space and resistance to breathing.
The wash-out phase is initiated by disconnecting the bias flow during expiration. The wash-out should continue until the end-tidal tracer gas concentration has fallen below 1/40th of the starting concentration over three successive breaths.
Guidelines on lung function testing recommend that the MBW test be repeated in order to obtain at least two tests in which the difference between two FRC values is less than 10% when comparing the higher to the lower FRC value.
In the conventional MBW test the FRC is calculated from the net volume of inert gas exhaled divided by the difference in end-tidal fractional concentration at the start and end of the wash-out:
                              F          ⁢                                          ⁢          R          ⁢                                          ⁢          C                =                              Net            ⁢                                                  ⁢            volume            ⁢                                                  ⁢            of            ⁢                                                  ⁢            inert            ⁢                                                  ⁢            gas            ⁢                                                  ⁢            exhaled                                              C                              ET                ,                start                                      -                          C                              ET                ,                end                                                                        (        2        )            
As the net volume of inert gas exhaled (numerator) is obtained by integration of the product of respiratory flow and fractional tracer gas concentration (i.e. expired and re-inspired tracer gas volumes on a breath-by-breath basis), accurate determination of the FRC requires a rapid dynamic response and data acquisition rate of the gas analyser. Proper alignment in time of the respiratory flow signal and fractional tracer gas concentration prior to the calculation is also critical. This makes demands on the performance of the gas analyser and the calibration of the equipment.